Copper electroplating compositions and methods of electroplating copper on substrates

ABSTRACT

Copper electroplating compositions which include a diimidazole compound enables the electroplating of copper having uniform morphology on substrates. The composition and methods of enable copper electroplating of photoresist defined features. Such features include pillars, bond pads and line space features.

FIELD OF THE INVENTION

The present invention is directed to copper electroplating compositionsand methods of electroplating copper on substrates, wherein the copperelectroplating compositions include a diimidazole compound to providecopper deposits having uniform morphology. More specifically, thepresent invention is directed to copper electroplating compositions andmethods of electroplating copper on substrates, wherein the copperelectroplating compositions include an diimidazole compound to providecopper deposits having uniform morphology and wherein the copperelectroplating compositions and copper electroplating methods can beused to electroplate photoresist defined features.

BACKGROUND OF THE INVENTION

Photoresist defined features include copper pillars and redistributionlayer wiring such as bond pads and line space features for integratedcircuit chips and printed circuit boards. The features are formed by theprocess of lithography where a photoresist is applied to a substratesuch as a semiconductor wafer chip often referred to as a die inpackaging technologies, or epoxy/glass printed circuit boards. Ingeneral, the photoresist is applied to a surface of the substrate and amask with a pattern is applied to the photoresist. The substrate withthe mask is exposed to radiation such as UV light. Typically thesections of the photoresist which are exposed to the radiation aredeveloped away or removed exposing the surface of the substrate.Depending on the specific pattern of the mask an outline of a circuitline or aperture may be formed with the unexposed photoresist left onthe substrate forming the walls of the circuit line pattern or aperture.The surface of the substrate includes a metal seed layer or otherconductive metal or metal alloy material which enables the surface ofthe substrate conductive. The substrate with the patterned photoresistis then immersed in a metal electroplating bath, typically a copperelectroplating bath, and metal is electroplated in the circuit linepattern or aperture to form features such as pillars, bond pads orcircuit lines, i.e., line space features. When electroplating iscomplete, the remainder of the photoresist is stripped from thesubstrate with a stripping solution and the substrate with thephotoresist defined features is further processed.

Pillars, such as copper pillars, are typically capped with solder toenable adhesion as well as electrical conduction between thesemiconductor chip to which the pillars are plated and a substrate. Sucharrangements are found in advanced packaging technologies. Solder cappedcopper pillar architectures are a fast growing segment in advancedpackaging applications due to improved input/output (I/O) densitycompared to solder bumping alone. A copper pillar bump with thestructure of a non-reflowable copper pillar and a reflowable solder caphas the following advantages: (1) copper has low electrical resistanceand high current density capability; (2) thermal conductivity of copperprovides more than three times the thermal conductivity of solder bumps;(3) can improve traditional BGA CTE (ball grid array coefficient ofthermal expansion) mismatch problems which can cause reliabilityproblems; and (4) copper pillars do not collapse during reflow allowingfor very fine pitch without compromising stand-off height.

Of all the copper pillar bump fabrication processes, electroplating isby far the most commercially viable process. In the actual industrialproduction, considering the cost and process conditions, electroplatingoffers mass productivity and there is no polishing or corrosion processto change the surface morphology of copper pillars after the formationof the copper pillars. Therefore, it is particularly important to obtaina smooth surface morphology by electroplating. The ideal copperelectroplating chemistry and method for electroplating copper pillarsyields deposits with excellent uniformity, flat pillar shape andvoid-free intermetallic interface after reflow with solder and is ableto plate at high deposition rates to enable high wafer through-out.However, development of such plating chemistry and method is a challengefor the industry as improvement in one attribute typically comes at theexpense of another. Copper pillar based structures have already beenemployed by various manufacturers for use in consumer products such assmart phones and PCs. As Wafer Level Processing (WLP) continues toevolve and adopt the use of copper pillar technology, there will beincreasing demand for copper plating baths and methods with advancedcapabilities that can produce reliable copper pillar structures.

Similar problems of morphology are also encountered with the metalelectroplating of redistribution layer wiring. Defects in the morphologyof bond pads and line space features also compromise the performance ofadvanced packaging articles. Accordingly, there is a need for copperelectroplating compositions and copper electroplating methods whichprovide copper deposits having uniform morphology and which can be usedto electroplate copper in the formation of photoresist defined features.

SUMMARY OF THE INVENTION

The present invention includes a composition including one or moresources of copper ions; one or more electrolytes; one or moreaccelerators; one or more suppressors; and one or more diimidazolecompounds having a formula:

wherein R₁, R₁, R₃ and R₄ are independently chosen from hydrogen; linearor branched (C₁-C₄)alkyl; and phenyl.

The present invention further includes a method including:

-   -   a) providing a substrate;    -   b) providing a copper electroplating composition including one        or more sources of copper ions; one or more electrolytes; one or        more accelerators; one or more suppressors; and one or more        diimidazole compounds having a formula:

-   -   -   wherein R₁, R₂, R₃ and R₄ are independently chosen from            hydrogen; linear or branched (C₁-C₄)alkyl; and phenyl;

    -   c) applying the copper electroplating composition to the        substrate; and

    -   d) electroplating copper having a uniform morphology on the        substrate with the copper electroplating composition.

The copper electroplating compositions of the present invention enablecopper deposits having uniform morphology and can be used to copperelectroplate photoresist features on substrates. The photoresistfeatures electroplated with the copper electroplating composition andmethod of the present invention have substantially uniform morphologyand are substantially free of nodules. Photoresist features such ascopper pillars and bond pads have a substantially flat profile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a 50× zoomed in 3D image of a 50 μm diameter×30 μm heightcopper pillar electroplated from a copper electroplating bath of thepresent invention containing 1,5-dihydrobenzo[1,2-d:4,5-d′]diimidazole.

FIG. 2 is a 50× zoomed in 3D image of a 50 μm diameter×30 μm heightcopper pillar electroplated from a comparative copper electroplatingbath containing3,3′-(ethane-1,2-diyl)bis(1-(2-hydroxyethyl)-1H-imidazol-3-ium)chloride.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout this specification the following abbreviations shallhave the following meanings unless the context clearly indicatesotherwise: A=amperes; A/dm²=amperes per square decimeter=ASD; °C.=degrees Centigrade; UV=ultraviolet radiation; g=gram; ppm=parts permillion=mg/L; L=micron=micrometer; mm=millimeters; cm=centimeters;DI=deionized; mL=milliliter; mol=moles; mmol=millimoles; Mw=weightaverage molecular weight; Mn=number average molecular weight;3D=three-dimensional; FIB=focus ion beam; WID=within−die; % WID=ameasure of height uniformity of pillars within a die; TIR=totalindicated runout=total indicator reading=full indicator movement=FIM;and RDL=redistribution layer.

As used throughout this specification, the term “plating” refers tocopper electroplating. “Deposition” and “plating” are usedinterchangeably throughout this specification. “Accelerator” refers toan organic additive that increases the plating rate of theelectroplating bath. “Suppressor” refers to an organic additive thatsuppresses the plating rate of a metal during electroplating. The term“array” means an ordered arrangement. The term “moiety” means a part ofa molecule or polymer that may include either whole functional groups orparts of functional groups as substructures. The terms “moiety” and“group” are used interchangeably throughout the specification. The term“aperture” means opening, hole, or gap. The term “morphology” means theform, shape and structure of an article. The term “total indicatorrunout” or “total indicator reading” is the difference between themaximum and minimum measurements, that is, readings of an indicator, onplanar, cylindrical, or contoured surface of a part, showing its amountof deviation from flatness, roundness (circularity), cylindricity,concentricity with other cylindrical features or similar conditions. Theterm “profilometry” means the use of a technique in the measurement andprofiling of an object or the use of a laser or white lightcomputer-generated projections to perform surface measurements of threedimensional objects. The term “pitch” means a frequency of featurepositions from each other on a substrate. The term “average” means anumber expressing the central value of a parameter, and the centralvalue is determined by adding the numerical values measured or collectedfor a particular parameter for a plurality of samples and dividing thesum of the values measured for each sample by the total number ofsamples. The term “parameter” means a numerical or other measurablefactor forming one of a set that defines a system or sets the conditionsof its operation. The term “circumference” means the border around apillar. The term “e.g.” means for example. The articles “a” and “an”refer to the singular and the plural.

All numerical ranges are inclusive and combinable in any order, exceptwhere it is clear that such numerical ranges are constrained to add upto 100%.

The present invention includes a composition including one or moresources of copper ions; and, corresponding anions of the one or moresources of copper ions (cations); one or more electrolytes; one or moreaccelerators; one or more suppressors; one or more imidazole compoundshaving a formula:

wherein R₁, R₂, R₃ and R₄ are independently chosen from hydrogen; linearor branched (C₁-C₄)alkyl; and phenyl; and the solvent is water.Preferably, R₁, R₂, R₃ and R₄ are independently chosen from hydrogen;and linear or branched (C₁-C₄)alkyl; more preferably, R₁, R₂, R₃ and R₄are independently chosen from hydrogen; and linear (C₁-C₂) alkyl; evenmore preferably, R₁, R₂, R₃ and R₄ are independently chosen fromhydrogen and methyl; most preferably R₁, R₂, R₃ and R₄ are hydrogen(1,5-dihydrobenzo[1,2-d:4,5-d′]diimidazole). Diimidazole compounds ofthe present invention have un-quaternized nitrogens. Such compounds canbe readily made accordingly the chemical literature or obtainedcommercially, such as from Sigma-Aldrich, Milwaukee, Wis., USA.

The one or more diimidazole compounds of the present invention can beincluded in the copper electroplating compositions in sufficient amountsto provide a copper deposit having smooth and uniform surfacemorphology. Preferably, the one or more diimidazole compounds of thepresent invention are included in the copper electroplating compositionsin amounts of 0.25 mg/L to 1000 mg/L (e.g. 0.5 mg/L to 800 mg/L, or suchas from 1 mg/L to 700 mg/L); more preferably, from 10 mg/L to 500 mg/L(e.g. 15 mg/L to 450 mg/L, or such as from 25 mg/L to 250 mg/L); evenmore preferably from 30 mg/L to 500 mg/L (e.g. 35 mg/L to 400 mg/L, orsuch as from 40 mg/L to 350 mg/L); most preferably from 40 mg/L to 200mg/L (e.g. 45 mg/L to 150 mg/L, or such as from 50 mg/L to 100 mg/L),based on the total weight of the copper electroplating composition.

The aqueous copper electroplating compositions include copper ions, fromone or more sources, such as water soluble copper salts. Such watersoluble copper salts include, but are not limited to, copper sulfate,such as copper sulfate pentahydrate; copper halides such as copperchloride; copper acetate; copper nitrate; copper tetrafluoroborate;copper alkylsulfonates; copper aryl sulfonates; copper sulfamate; copperperchlorate and copper gluconate. Exemplary copper alkane sulfonatesinclude copper (C₁-C₆)alkane sulfonate and more preferably copper(C₁-C₃)alkane sulfonate. Preferred copper alkane sulfonates are coppermethanesulfonate, copper ethanesulfonate and copper propanesulfonate.Exemplary copper arylsulfonates include, but are not limited to, copperbenzenesulfonate and copper p-toluenesulfonate. Mixtures of copper ionsources may be used. Such copper salts are well known to those of skillin the art or can be readily made accordingly the chemical literature orobtained commercially, such as from Sigma-Aldrich. In addition to thecopper ions (cations), the copper electroplating compositions includethe corresponding anions of the water soluble copper salts. The copperelectroplating compositions of the present invention are free of othermetals, such as alloying metals, with the exception of unavoidableimpurities. One or more water soluble copper salts are included in thecopper electroplating compositions of the present invention in amountsto provide a copper deposit having smooth and uniform copper morphology.Preferably, one or more of the copper salts are present in amountssufficient to provide copper ion concentrations of 30 g/L to 70 g/L ofplating solution; more preferably, at concentrations of 40 g/L to 60g/L.

Electrolytes of the present invention can be alkaline or acidic.Preferably the electrolyte is acidic. Preferably, the pH of theelectrolyte is ≤2; more preferably the pH is ≤1. Acidic electrolytesinclude, but are not limited to, sulfuric acid, acetic acid, fluoroboricacid, alkanesulfonic acids such as methanesulfonic acid, ethanesulfonicacid, propanesulfonic acid and trifluoromethane sulfonic acid, arylsulfonic acids such as benzenesulfonic acid, p-toluenesulfonic acid,sulfamic acid, hydrochloric acid, hydrobromic acid, perchloric acid,nitric acid, chromic acid and phosphoric acid. Mixtures of acids can beused in the present copper plating compositions. Preferred acids includesulfuric acid, methanesulfonic acid, ethanesulfonic acid,propanesulfonic acid, hydrochloric acid and mixtures thereof. The acidsmay be present in an amount in the range of 1 to 400 g/L. Electrolytesare generally commercially available from a variety of sources and canbe used without further purification.

Optionally, electrolytes of the present invention can contain a sourceof halide ions. Preferably, chloride ions and bromide ions are used;more preferably, chloride ions are included in the copper electroplatingcompositions. Exemplary chloride ion sources include copper chloride,sodium chloride, potassium chloride and hydrochloric acid (hydrogenchloride). Sources of bromide ions include sodium bromide, potassiumbromide and hydrogen bromide. A wide range of halide ion concentrationsmay be used in the present invention. Preferably, the halide ionconcentration is in the range of 0.5 mg/L to 200 mg/L based on theplating composition; more preferably, form 10 mg/L to 150 mg/L; mostpreferably, from 50 mg/L to 100 mg/L. Such halide ion sources aregenerally commercially available and can be used without furtherpurification.

Accelerators (also referred to as brightening agents) include, but arenot limited to, N,N-dimethyl-dithiocarbamic acid-(3-sulfopropyl)ester;3-mercapto-propylsulfonic acid-(3-sulfopropyl)ester;3-mercapto-propylsulfonic acid sodium salt; carbonic acid,dithio-O-ethylester-S-ester with 3-mercapto-1-propane sulfonic acidpotassium salt; bis-sulfopropyl disulfide; bis-(sodiumsulfopropyl)-disulfide; 3-(benzothiazolyl-S-thio)propyl sulfonic acidsodium salt; pyridinium propyl sulfobetaine;1-sodium-3-mercaptopropane-1-sulfonate; N,N-dimethyl-dithiocarbamicacid-(3-sulfoethyl)ester; 3-mercapto-ethyl propylsulfonicacid-(3-sulfoethyl)ester; 3-mercapto-ethylsulfonic acid sodium salt;carbonic acid-dithio-O-ethylester-S-ester with 3-mercapto-1-ethanesulfonic acid potassium salt; bis-sulfoethyl disulfide;3-(benzothiazolyl-S-thio)ethyl sulfonic acid sodium salt; pyridiniumethyl sulfobetaine; and 1-sodium-3-mercaptoethane-1-sulfonate. Suchaccelerators are commercially available, such as from Sigma-Aldrich, orcan be made according to the chemical literature. Accelerators can beused in a variety of concentrations. Preferably, accelerators are usedin amounts of 0.1 mg/L to 1000 mg/L; more preferably, from 0.5 mg/L to500 mg/L; most preferably from 1 mg/L to 50 mg/L.

Suppressors include, but are not limited to, polypropylene glycolcopolymers and polyethylene glycol copolymers, including ethyleneoxide-propylene oxide (“EO/PO”) copolymers and butyl alcohol-ethyleneoxide-propylene oxide copolymers. The weight average molecular weight ofthe suppressors may range from 800-15000, preferably from 1000 to15,000. Suppressors are included in amounts of 0.5 g/L to 15 g/L basedon the weight of the plating composition; preferably, from 0.5 g/L to 5g/L.

The copper electroplating compositions can be prepared by combining thecomponents in any order. It is preferred that the inorganic componentssuch as source of copper ions, water, electrolyte and optional halideion source are first added to the bath vessel, followed by the organiccomponents such as imidazole compounds, accelerator, suppressor, and anyother organic component.

Optionally, the aqueous copper electroplating baths can include aconventional leveling agent provided such the leveling agent does notsubstantially compromise the morphology of the copper deposits. Suchleveling agents may include those disclosed in U.S. Pat. No. 6,610,192to Step et al., U.S. Pat. No. 7,128,822 to Wang et al., U.S. Pat. No.7,374,652 to Hayashi et al. and U.S. Pat. No. 6,800,188 to Hagiwara etal. Such leveling agents can be included in conventional amounts;however, it is preferred that such leveling agents are excluded from thecopper electroplating compositions of the present invention.

Optionally, the copper electroplating compositions of the presentinvention can include additives such as buffers to assist in maintaininga desired pH; antimicrobial agents; surfactants such as non-ionic,cationic, anionic and zwitterion surfactants; preferably, thesurfactants are non-ionic surfactants; and de-foaming agents. Suchadditives are well known to those of skill in the art and are used intheir conventional amounts or minor experimentation can be carried outto determine the optimum concentrations of the additives to be includedin the copper electroplating compositions of the present invention.

Preferably, the copper electroplating compositions consist of one ormore sources of copper ions; and, corresponding anions of the one ormore sources of copper ions (cations); one or more electrolytes; one ormore accelerators; one or more suppressors; one or more diimidazolecompounds having formula (I); water; and, optionally, one or moresources of halide ions; and one or more additives chosen from buffers,antimicrobial agents, surfactants, and de-foaming agents.

More preferably, the copper electroplating compositions consist of oneor more sources of copper ions; and, corresponding anions of the one ormore sources of copper ions (cations); one or more electrolytes; one ormore accelerators; one or more suppressors;1,5-dihydrobenzo[1,2-d:4,5-d′]diimidazole; water; and, optionally, oneor more sources of halide ions; and one or more additives chosen frombuffers, antimicrobial agents, surfactants, and de-foaming agents.

The copper electroplating compositions of the present invention can beused to electroplate copper at temperatures of 10° C. to 65° C.Preferably, the temperature of the plating composition is from 15 to 50°C.; more preferably from room temperature to 40° C.

Preferably, the copper electroplating compositions are agitated duringplating. Any suitable agitation method can be used. Methods of agitatingare well-known in the art. Such agitation methods include, but are notlimited to, air sparging, work piece agitation, and impingement.

A substrate can be electroplated with copper by contacting the substratewith the copper electroplating composition of the present invention. Thesubstrate can function as a cathode. The anode can be a soluble anode,such as a copper anode, or an insoluble anode. Various insoluble anodesare known to those of skill in the art. Electrical potential is appliedto the electrodes. Current densities can range from 0.25 ASD to 40 ASD;preferably, 1 ASD to 30 ASD; more preferably, from 10 ASD to 30 ASD.

The copper electroplating compositions and methods of the presentinvention can be used to plate copper having a smooth and uniformmorphology on various substrates where smooth and uniform morphologycopper deposits are desired; however, the copper electroplatingcompositions and methods are preferably used to plate photoresistdefined features.

Methods and compositions for electroplating copper photoresist definedfeatures of the present invention enable an array of photoresist definedfeatures having an average TIR such that the features have a morphologywhich is substantially smooth, free of nodules and with respect topillars, bond pads and line space features have substantially flatprofiles. The photoresist defined features of the present invention areelectroplated with photoresist remaining on the substrate and extendbeyond the plane of the substrate. This is in contrast to dual damasceneand printed circuit board plating which do not use photoresist to definefeatures which extend beyond the plane of the substrate but are inlaidinto the substrate. An important difference between photoresist definedfeatures and damascene and printed circuit board features is that withrespect to the damascene and printed circuit boards the plating surfaceincluding the sidewalls are all conductive. The dual damascene andprinted circuit board plating baths have a bath formulation thatprovides bottom-up or super-conformal filling, with the bottom of thefeature plating faster than the top of the feature. In photoresistdefined features, the sidewalls are non-conductive photoresist andplating only occurs at the feature bottom with the conductive seed layerand proceeds in a conformal or same plating speed everywhere deposition.

While the present invention is substantially described with respect tomethods of electroplating copper pillars having a circular morphology,the present invention also applies to other photoresist defined featuressuch as bond pads and line space features. In general, the shapes of thefeatures may be, for example, oblong, octagonal and rectangular inaddition to circular or cylindrical. The methods of the presentinvention are preferably for electroplating copper cylindrical pillars,wherein the pillars have a substantially flat top.

The copper electroplating methods provide an array of copper photoresistdefined features, such as copper pillars, with an average TIR of −3 to3; preferably, from −2 to 2; more preferably from −2 to 1; mostpreferably from −2 to −1.

The average TIR for an array of photoresist defined features on asubstrate involves determining the TIR of individual features from thearray of features on the single substrate and averaging them. Theaverage TIR for features of a given substrate can be determined bydetermining the TW for individual features of a region of low density,medium density or high density pitch, or combinations thereof, addingthe measured values and averaging the values. By measuring the TIR of avariety of individual features, the average TIR becomes representativeof the whole substrate.

Individual feature TIRs may be determined by the following equation:

TIR=height_(center)−height_(edge),

where height_(center) is the height of a pillar as measured along itscenter axis and height_(edge) is the height of the pillar as measuredalong its edge at the highest point on the edge.

In addition, the copper electroplating methods and compositions canprovide an array of copper photoresist defined features with a % WID of0% to 16%; preferably, from 5% to 16%; more preferably, from 12% to 16%;most preferably, from 14% to 16%. The % WID or within-die can bedetermined by the following equation:

% WID=½×[(height_(max)−height_(min))/height_(avg)]×100

where height_(max) is the height of the tallest pillar of an array ofpillars electroplated on a substrate as measured at the tallest part ofthe pillar. Height_(min) is the height of the shortest pillar of anarray of pillars electroplated on the substrate as measured at thetallest part of the pillar. Height_(avg) is the average height of all ofthe pillars electroplated on the substrate. Most preferably, the copperelectroplating compositions and methods of the present invention providean array of photoresist defined features on a substrate where there is abalance between the average TIR and % WID such that the average TIRranges from −3 to 3 and the % WID ranges from 0% to 16% with thepreferred ranges for each parameter as disclosed above.

The parameters of the pillars for determining TIR and % WID may bemeasured using optical profilometry such as with a KEYENCE 3D LaserScanning Confocal Microscope VK-X Series or similar apparatus such as awhite light LEICA DCM 3D. Parameters such as pillar height and pitch maybe measured using such devices.

The copper pillars electroplated from the copper electroplatingcompositions of the present invention can have aspect ratios of 3:1 to1:1 or such as 2:1 to 1:1. RDL type structure may have aspect ratios aslarge as 1:20 (height:width).

While the method of the present invention may be used to electroplatephotoresist defined features such as pillars, bonding pads and linespace features, the method is described in the context of plating copperpillars which is the preferred feature of the present invention. Thecopper pillars of the present invention can be formed by firstdepositing a conductive seed layer on a substrate such as asemiconductor chip or die. The substrate is then coated with aphotoresist material and imaged to selectively expose the photoresistlayer to radiation, such as UV radiation. The photoresist layer may beapplied to a surface of the semiconductor chip by conventional processesknown in the art. The thickness of the photoresist layer may varydepending on the height of the features. The thickness can range from 1μm to 350 μm; preferably, from 10 μm to 230 μm; more preferably, from 20μm to 220 μm. A patterned mask is applied to a surface of thephotoresist layer. The photoresist layer may be a positive or negativeacting photoresist. When the photoresist is positive acting, theportions of the photoresist exposed to the radiation are removed with adeveloper such as an alkaline developer. A pattern of a plurality ofapertures such as vias is formed on the surface which reaches all theway down to the seed layer on the substrate or die. The pitch of thepillars can range from 20 μm to 400 μm; preferably, the pitch may rangefrom 100 μm to 350 μm; more preferably, the pitch of the pillars canrange from 100 μm to 250 μm. The diameters of the vias can varydepending on the diameters of the features (pillars). The diameters ofthe vias can range from 2 μm to 300 μm; preferably, from 5 μm to 225 μm;more preferably, from 15 μm to 200 μm. The entire structure can then beplaced in a copper electroplating composition of the present invention.Electroplating is done to fill at least a portion of each via with acopper pillar with a substantially flat top. The electroplating isconformal or same plating speed everywhere deposition, notsuper-conformal or superfilling. The entire structure with the copperpillars is then transferred to a bath containing solder, such as a tinsolder or tin alloy solder such as a tin/silver or tin/lead alloy and asolder bump is electroplated on the substantially flat surface of eachcopper pillar to fill portions of the vias. The remainder of thephotoresist is removed by conventional means known in the art leaving anarray of copper pillars with solder bumps on the die. The remainder ofthe seed layer not covered by pillars is removed through etchingprocesses well known in the art. The copper pillars with the solderbumps are placed in contact with metal contacts of a substrate such as aprinted circuit board, another wafer or die or an interposer which canbe made of organic laminates, silicon or glass. The solder bumps areheated by conventional processes known in the art to reflow the solderand join the copper pillars to the metal contacts of the substrate.Conventional reflow processes for reflowing solder bumps can be used. Anexample of a reflow oven is FALCON 8500 tool from Sikiama International,Inc. which includes 5 heating and 2 cooling zones. Reflow cycles mayrange from 1-5. The copper pillars are both physically and electricallycontacted to the metal contacts of the substrate. An underfill materialmay then be injected to fill space between the die, the pillars and thesubstrate. Conventional underfills which are well known in the art canbe used.

FIG. 1 is a 3D image collected using a KEYENCE 3D Laser ScanningConfocal Microscope VK-X Series of a copper pillar of the presentinvention having cylindrical morphology with a base and flat top surfacemorphology for electroplating solder bumps. During reflow solder ismelted to obtain a smooth surface. If pillars are too domed duringreflow, the solder may melt and flow off the sides of the pillar andthen there is not enough solder on the top of the pillar for subsequentbonding steps, as shown in FIG. 2 which was also a 3D image as FIG. 1.If the pillar is too dished, or has a sink-hole type configuration,material left from the copper bath which was used to electroplate thepillar can be retained in the dished top and contaminate the solderbath, thus shortening the life of the solder bath.

To provide a metal contact and adhesion between the copper pillars andthe semiconductor die during electroplating of the pillars, an underbumpmetallization layer typically composed of a material such as titanium,titanium-tungsten or chromium is deposited on the die. Alternatively, ametal seed layer, such as a copper seed layer, may be deposited on thesemiconductor die to provide metal contact between the copper pillarsand the semiconductor die. After the photosensitive layer has beenremoved from the die, all portions of the underbump metallization layeror seed layer are removed except for the portions underneath thepillars. Conventional processes known in the art can be used.

While the height of the copper pillars can vary, they preferably, rangein height from 1 μm to 300 μm; more preferably, from 5 μm to 225 μm;even more preferably from 15 μm to 200 μm. Diameters of the copperpillars can also vary. Preferably, the copper pillars have a diameter of2 μm to 300 μm; more preferably, from 5 μm to 225 μm; even morepreferably, 15 μm to 200 μm.

The copper electroplating methods and compositions provide copperphotoresist defined features which have a substantially uniformmorphology and are substantially free of nodules. The copper pillars andbond pads have a substantially flat profile. The copper electroplatingcomposition and methods enable an average TIR to achieve the desiredmorphology as well as a balance between an average TW and % WID.

The following examples are intended to further illustrate the inventionbut are not intended to limit its scope.

Example 1 (Invention) Copper Electroplating Bath

The following copper electroplating bath of the present invention wasprepared with the components and amounts as disclosed in Table 1 below.

TABLE 1 COMPONENT AMOUNT Copper ions from copper sulfate 50 g/Lpentahydrate 1,5-dihydrobenzo[1,2-d:4,5-d′]diimidazole 50 mg/L Sulfuricacid (98%) 100 g/L Chloride ions from hydrogen chloride 90 mg/Lbis-(sodium sulfopropyl)-disulfide 6 mg/L EO/PO copolymer having aweight average 0.5 g/L molecular weight of 1,000 and terminal hydroxylgroups Water To one literThe components of the copper electroplating bath were mixed together atroom temperature with stirring. The pH of the copper electroplating bathwas <1.

Example 2 (Comparative with Quaternized Nitrogens) Synthesis of3,3′-(ethane-1,2-diyl)bis(1-(2-hydroxyethyl)-1H-imidazol-3-ium) chloride

N-(2-Hydroxyethyl)imidazole (2.55 g, 22.7 mmol) and 1,2-dichloroethane(1.00 g, 10.11 mmol) were weighed into a 20-mL pressure tube.Acetonitrile (10 mL) was added, the tube was sealed and heated to 90° C.for 60 hours. Cooled to room temperature and the resulting precipitatewas isolated by filtration, washed with fresh acetonitrile, and dried invacuo, giving 2.91 g (59%) of the compound as a white powder.

1H NMR (400 MHz, DMSO-d6) δ 9.37 (s, 2H), 7.79 (s, 4H), 5.50 (t, J=5.5Hz, 2H), 4.81 (s, 4H), 4.23 (t, J=4.6 Hz, 4H), 3.70 (t, J=5.2 Hz, 4H).13C NMR (101 MHz, DMSO-d6) δ 136.96, 123.06, 122.28, 59.08, 51.84,48.30.

Example 3 (Comparative) Copper Electroplating Bath

The following copper electroplating bath of the present invention wasprepared with the components and amounts as disclosed in Table 2 below.

TABLE 2 COMPONENT AMOUNT Copper ions from copper sulfate 50 g/Lpentahydrate 3,3′-(ethane-1,2-diyl)bis(1-(2- 300 mg/Lhydroxyethyl)-1H-imidazol-3-ium) chloride Sulfuric acid (98%) 100 g/LChloride ions from hydrogen chloride 90 mg/L bis-(sodiumsulfopropyl)-disulfide 6 mg/L EO/PO copolymer having a weight average0.5 g/L molecular weight of 1,000 and terminal hydroxyl groups Water Toone literThe components of the copper electroplating bath were mixed together atroom temperature with stirring. The pH of the copper electroplating bathwas <1.

Example 4 (Invention)

A 300 mm silicon wafer die with two different pitch areas (densepitch=100 μm, and sparse pitch=250 μm), wherein each area had patternedphotoresist 50 μm thick and a plurality of apertures with diameters of50 μm in each area (available from IMAT, Inc., Vancouver, Wash.) wasimmersed in the copper electroplating bath of the present invention asdisclosed in Table 1 of Example 1. The anode was a soluble copperelectrode. The wafer and the anode were connected to a rectifier andcopper pillars were electroplated on the exposed seed layer at thebottom of the apertures. Average current density during plating was 15ASD and the temperature of the copper electroplating bath was at 25° C.The pH of the plating bath was <1. After electroplating the remainingphotoresist was then stripped with BPR photostripper solution availablefrom the Dow Chemical Company leaving an array of copper pillars on thewafer in the two different pitch areas. Eight copper pillars from eacharea were then analyzed for their morphology. The heights at the centerand edge of the copper pillars and TIR of the pillars were measuredusing a KEYENCE 3D Laser Scanning Confocal Microscope VK-X Series. TheTIR was determined by the following equation:

TIR=height_(center)−height_(edge)

The average TW of the eight pillars was also determined as shown in theTable 3.

TABLE 3 Variable Pitches Pillar Height_(max) Pillar TIR for Each Pillar# (μm) (μm) Pillar# (μm) 1 100 36.0 −1.2 2 100 30.5 −1.6 3 100 28.9 −1.64 100 27.2 −1.6 5 100 27.6 −1.7 6 250 36.9 −1.4 7 250 35.3 −1.4 8 25031.3 −1.8 Average — 31.7 −1.6

The % WID for the array of pillars was determined with a KEYENCE 3DLaser Scanning Confocal Microscope VK-X Series and the followingequation:

% WID=½×[(height_(max)−height_(min))/height_(avg)]×100

The % WID across the dense and the sparse pitches (i.e. 8 pillarsmeasured across the dense and sparse pitches) was 15.4% and the averageTIR was −1.6. The surface of the pillars all appeared smooth and free ofnodules. The copper electroplating bath which included reaction product1 plated very good copper pillars. FIG. 1 is an image of pillar 4 frompitch 100 μm plated on a seed layer and analyzed using the 3D imagecollected using a KEYENCE 3D Laser Scanning Confocal Microscope VK-XSeries. The surface morphology was smooth and flat on top suitable forreceiving solder.

Example 5 (Comparative)

A 300 mm silicon wafer die with two different pitch areas (densepitch=100 μm, and sparse pitch=250 μm), wherein each area had patternedphotoresist 50 μm thick and a plurality of apertures with diameters of50 μm in each area (available from IMAT, Inc., Vancouver, Wash.) wasimmersed in the comparative copper electroplating bath as disclosed inTable 2 of Example 3 with the quaternized nitrogen compound. The anodewas a soluble copper electrode. The wafer and the anode were connectedto a rectifier and copper pillars were electroplated on the exposed seedlayer at the bottom of the apertures. Average current density duringplating was 15 ASD and the temperature of the copper electroplating bathwas at 25° C. The pH of the plating bath was <1. After electroplatingthe remaining photoresist was then stripped with BPR photostrippersolution available from the Dow Chemical Company leaving an array ofcopper pillars on the wafer in the two different areas. Eight copperpillars from each area were then analyzed for their morphology. Theheights at the center and edge of the copper pillars and TIR of thepillars were measured using a KEYENCE 3D Laser Scanning ConfocalMicroscope VK-X Series. The TIR was determined by the followingequation: TIR=height_(center)−height_(edge)

The average TIR of the eight pillars was also determined as shown in theTable 3.

TABLE 3 Variable Pitches Pillar Height_(max) Pillar TIR for Each Pillar# (μm) (μm) Pillar# (μm) 1 100 39.4 5.3 2 100 32.6 3.9 3 100 31.4 4.2 4100 29.0 3.6 5 100 29.6 4.3 6 250 41.4 4.6 7 250 39.3 4.7 8 250 34.4 4.5Average — 34.6 4.4

The % WID for the array of pillars was determined with a KEYENCE 3DLaser Scanning Confocal Microscope VK-X Series and the followingequation:

% WID=½×[(height_(max)−height_(min))/height_(avg)]×100

The % WID across dense and sparse pitches was 17.9% (i.e. 8 pillarsmeasured across the dense and sparse pitches) and the average TIR was+4.4. The tops of the pillars appeared domed and rough, thus unsuitablefor receiving solder. FIG. 2 is an image of pillar 5 from pitch 100 μmplated on a seed layer and analyzed using the 3D image collected with aKEYENCE 3D Laser Scanning Confocal Microscope VK-X Series. The surfacemorphology of the circumference of the pillar appeared smooth; however,the top was rounded and rough and unsuitable for receiving solder.

What is claimed is:
 1. L A composition comprising one or more sources ofcopper ions; one or more electrolytes; one or more accelerators; one ormore suppressors; and one or more imidazole compounds having a formula:

wherein R₁, R₂, R₃ and R₄ are independently chosen from hydrogen; linearor branched (C₁-C₄)alkyl; and phenyl.
 2. The composition of claim 1,wherein the one or more imidazole compounds are in amounts of 0.25 ppmto 1000 ppm.
 3. The composition of claim 1, wherein R₁, R₂, R₃ and R₄are independently chosen from hydrogen; and (C₁-C₂)alkyl.
 4. Thecomposition of claim 3, wherein R₁, R₂, R₃ and R₄ are independentlychosen from hydrogen; and methyl.
 5. A method comprising: a) providing asubstrate; b) providing a copper electroplating composition comprisingone or more sources of copper ions; one or more electrolytes; one ormore accelerators; one or more suppressors; and one or more imidazolecompounds having a formula:

wherein R₁, R₂, R₃ and R₄ are independently chosen from hydrogen; linearor branched (C₁-C₄)alkyl; and phenyl; c) applying the copperelectroplating composition to the substrate; and d) electroplatingcopper having a uniform morphology on the substrate with the copperelectroplating composition.
 6. The method of claim 1, wherein thesubstrate comprises photoresist defined features and the photoresistdefined features are electroplated with copper during electroplating. 7.The method of claim 6, wherein the photoresist defined features on thesubstrate are chosen from one or more of pillars, bond pads and linespace features.
 8. The method of claim 5, wherein the one or moreimidazole compounds are in amounts of 0.25 ppm to 1000 ppm.
 9. Themethod of claim 5, wherein electroplating is done at a current densityof 0.25 ASD to 40 ASD.